Ancestors of a 21st century British family

According to Sir Archibald Edmonstone, 3rd Bt, the Archibald who founded the family of Duntreath was probably his [John Edmonstone?s] younger son. The Edmonstones of Duntreath descend from Archibald, believed to have been the second son of the first Sir John Edmonstone, and therefore brother of the second John, who married Isabella, daughter of Robert II.